The present invention concerns a process for authentication of a smart card and a system for use of the process.
In a pay-television network, such as that described by the French patent applications nos. 88 06121 and 87 17092, corresponding to U.S. Application Ser. No. 07/811,906 the subscribers' decoders, in order to unscramble coded broadcasts, require so-called "smart cards". To limit the risks of fraudulent use, it can be arranged to change these cards at intervals, for example every three months, "invalidating" the expiring cards by sending an appropriate signal to a given input of the card. However, these networks are not intended to detect the use of counterfeit cards or card simulators.
The FIG. 1 shows the block diagram of the pay television program scrambler according to U.S. patent application Ser. No. 811,906. The transmitter and the television receiver 1 are briefly described as most of these items are well known per se.
The transmitter 1 comprises a source 2 of images supplying composite video signals. Its output is connected via an analog-digital converter 3 to an intermediate video scrambler circuit 4 of a type which is known per se. This circuit 4 is controlled by a control unit 5 via a pseudo-random sequence generator 6, and it sends to this central unit synchronization signals corresponding to the video signals. The circuit 4 is connected via a digital-analog converter 7 to a power transmitter 8 whose transmitting antenna is referenced 9.
In FIG. 1, there has been shown the block diagram of that one of the television receivers which is able to receive the transmissions from the transmitter 1 and provided with unscrambling circuits. This receiver is referenced 10. The receiver 10 comprises a receiving antenna 11 connected to a tuner 12 followed by an analog-digital converter 13, a digital video unscrambling circuit 14, and a digital/analog convertor 15 at the output 16 of which the unscrambled video signals are taken.
The output of the circuit 12 is also connected, via an amplifier 17, a filter 18 and a peak value detector 19, to the serial input of a shift register 20. The parallel outputs of the register 20 are connected, via a buffer register 21, to the data inputs of a microprocessor computer 22, for example a microprocessor of the EF6805 type.
The computer 22 is connected in a bidirectional manner to a security code checker 23. This checker 23 is connected to a "smart card" reader 24. The computer 22 is also connected to a pseudo-random sequence generator 25 which is itself connected to the circuit 14.
In the transmitter 1, the video signals coming from the source 2 and digitized by the converter 3 are scrambled in the circuit 4 under the control of pseudo-random sequences of signals produced by the generator 6. Each of the successive binary numbers of a sequence appearing in synchronism with the successive lines of the video signal determines a break point in the corresponding line, this break point being able to be located at any position in the line. The scrambling consists in permutating the sections of the line situated on either side of this break point. This scrambling method, called "scrambling by line permutation", being well known per se, will not be described in greater detail.
The sequences of the pseudo-random generator 6 have a relatively short cycle: 2.56, 5.12 or 10.24 seconds. The 2.56 s cycle is particularly advantageous for limiting the lock-on time of the decoder on switching on. The different values of sequences mentioned above are selected dynamically in the transmitter. These sequences are controlled by the control unit 5 as a function of an access to the program message, here called the ECM (Entitlement Checking Message), common to all the receivers. In some other pay television systems, the authorization messages are communicated to subscribers by post, by modem or by any other telecommunications means. The access messages can therefore be modified only at relatively long time intervals (usually a few weeks), which allows "pirates" to discover them (it is estimated that these messages can be discovered in 1 to 2 days) well before their next modification.
According to the disclosure of the application 811,906, the control unit 5 of the transmitter inserts access messages in the composite video (at the input of the converter 7). This central processing unit 5 (sic) provides, among other things, an image counter function (FCNT). This counter is incremented by unity every other frame, that is to say at each complete image (every 40 ms for a 50 Hz interlaced frame standard), using the image sync pulses of the video signal sent by the circuit 4. In the present case, this counter has a maximum count state of 255 (8-bit counter). After having reached this maximum state, the counter returns to zero, and allows the synchronization of control words (CW) whose encoded version is the said access message. This control word can, for example, have a length of 60 bits and it is chosen in a random manner. This control word determines a new cycle of the generator 6. Furthermore, the control unit 5 sends to the circuit 7, at specific instants, the following data: the state of the counter (FCNT), access message headers (H1 and H2), data concerning the state of the transmission system (scrambled or not scrambled, free access or pay access, message data rate), and check data ("check num" or CRC, for example). The central unit sends these data during the transmission of lines which are not used by the image. According to the CCETT 625-line standard, there are lines which are not used by the visible image before the 23rd and after the 310th line. In the present example, four of these lines not used by the image are used for the encoding, for example the lines 12 to 15 for a field F1 and the lines 325 to 328 for a field F2. At each of these lines, during the useful duration of the line, the central processing unit sends five bytes of encoding data, that is 40 bits per line and 20 bytes for each field of four lines. The binary zeros correspond to the black level and the "1"'s correspond to the white level. Thus at the start of each complete image (every 40 ms in the above-mentioned example), a data packet (of 40 bytes) is incorporated in the composite video signal transmitted by the transmitter 1.
Two different types of data packets are transmitted: ECM1 and ECM2. Advantageously, these data packets are encoded, for example, in 8-4 Hamming code, in order to be better protected from transmission interference. According to a variant, the different bytes of each packet can be interlaced within that packet in order to improve immunity from interference. They are all, of course, correspondingly de-interlaced in each decoder.